The present invention relates to apparatus and methods for improving the resolving power of mass analysers.
It is known that the dominant aberration that limits the resolution of high performance time of flight (TOF) mass analysers is due to the ion ‘turn around time’. The ultimate resolution of a TOF mass analyser is therefore fundamentally limited to a value that is inversely proportional to the orthogonal velocity spread of the ions being analysed and so it is desirable to reduce this as much as possible. Other aberrations are less difficult to correct, such as spatial focusing, mechanical tolerances and detection line width.
The aberration due to the ion turn around time in a TOF mass analyser, dTOF, may be defined as dTOFF=2u/a, where ‘u’ is the initial orthogonal velocity component of an ion and ‘a’ is the acceleration provided to the ion by the electric field generated by the pusher in the extraction region of the TOF mass analyser. The aberration due to the ion turn around time can therefore be reduced by reducing the initial velocity component of the ion ‘u’ or by increasing the acceleration of the ion ‘a’. A known technique to reduce the initial velocity component ‘u’ and hence reduce the ion turn around time involves the use of an Einzel transfer lens positioned upstream of the pusher assembly of the TOF mass analyser. The lens is designed to magnify the physical size of the ion beam whilst reducing its orthogonal velocity component. However, this system has the disadvantage of requiring a relatively large region for defining the pusher electric field.
It is also possible to reduce the orthogonal velocity spread of ions by using a mechanical slit to restrict the passage of ions with high orthogonal velocity ‘u’ to the pusher region. The ions may be directed towards the slit along an axis through the slit. By the time that the ions reach the slit only the ions having relatively low orthogonal velocities will remain close enough to the axis to pass through the slit and be sampled by the pusher electrode of the TOF mass analyser. The other ions are blocked by the slit. This method of rejecting ions by collimating the ion beam has the advantage of increasing the TOF mass resolution of the ions transmitted through the slit, but has the disadvantage of reducing the instrument sensitivity since the ions that are blocked by the slit cannot be mass analysed. Furthermore, if the axial energy of the ions is not high enough then electrical potentials on the surfaces of the slit may affect the passage of ions that pass through the slit.
Another technique for reducing the turn around time is to use a greater electric field to increase the acceleration of the ions ‘a’. This may be achieved by applying higher voltages to the pusher electrodes of the TOF mass analyser or by applying the same potential difference over a shorter length. However, higher voltage supplies use and dissipate more power and are more expensive. Also, the application of relatively high potential differences over relatively short lengths can lead to electrical breakdowns.
It is therefore desired to provide an improved mass spectrometer and improved method of mass spectrometry.